OCt, 20, 1938
SOLVENT P R O P E R T I E S O F L I Q U I D S : [CONTRIBUTION FROM
I < E L A T I O N TO
THESTATE UNIVERSITY
FRACTIONATION OF LIGNIN
5061
OF K E W YORK, COLLEGE O F FORESTRY]
The Solvent Properties of Liquids and Their Relation to the Solubility, Swelling, Isolation and Fractionation of Lignin BY CONRAD SCHUERCH RECEIVED MAY2 , 1952 The ability of solvents to dissolve or swell a variety of isolated lignins increases as the hydrogen-bonding capacities of the solvents increase and as their solubility parameters (after Hildebrand) approach a value of around eleven. Lower molecular weight lignin fractions are soluble in solvents with a wider range of solubility parameter and hydrogen bonding capacity than are the higher fractions. Earlier fractionations of ethanol and hydrol lignins are applications of this fact. Solubility trends in mixed solvents and within a homologous series of solvents are correlated with the same solvent properties. The yield of ethanol spruce lignin is substantially increased when the reaction is carried out in a better solvent system such as chloroformethanol. A threefold ethanolysis under mild conditions of acidity and temperature in this solvent mixture has given rise to a variety of products with widely different partition coefficients, solubilities and viscosities. The insolubility of lignin in wood and the heterogeneity and low molecular weight of its degradation products suggest the probability of a network structure or structures for lignin.
Although considerable data are available on the solubility of various isolated lignins, no general explanation of the subject seems to have appeared. Lignin solubility, compatibilities and swelling have therefore usually been investigated as isolated phenomena and similarities between many known lignin isolation methods have been obscured.’ This article suggests that the ability of solvents to dissolve or swell a variety of isolated lignins increases as the hydrogen-bonding capacities2of the solvents increase and as their solubility parameters3 approach a value of around eleven. The lignins investigated, all of which conform to this pattern, include commercial pine kraft lignin,4 commercial soda lignins,j ethanol maple lignin,e hydrol maple lignin,’ ethanol spruce lignin,* “native” spruce9 and aspen lignins.l0 The general similarity of the solubilities of these materials can be seen in Table I. The solubility of lignosulfonic acids, however, is completely altered by the introduction of the hydrophilic sulfonic acid group and is not comparable. I t is possible by mixing certain solvents with the lower alcohols to approximate the suggested conditions of maximum lignin solubility. If such a mixture is used as a solvent during alcoholysis, the yield of soluble alcohol lignin is markedly increased over that usually obtained.’ The ethanolysis of sprucewood in chloroform-ethanol has been carried out as an example of this technique and will be described in detail later in this article. Let us consider first the relationship of the solubility parameter of liquids to the solubility of lignin. Hildebrand has identified a constant which he calls ( 1 ) C. Schuerch. THIS JOURNAL, 73,2385 (1951). ( 2 ) W. Gordy, J . Chon. Phyr., 9, 204, 215 (1941). See also ref. 3, chapter 10, and M. L. Huggius, J. Ovg. C h e w . , 1, 407 (1936). ( 3 ) 1. H. Hildebrand and R. L. Scott. “Solubilitv of Non-electrolytes,” 3rd ed , Reinhold Publishing Corp., New York, N. Y., 1951. (4) ”Indulin.” described in Bulletin L-5 Industrial Chemical Sales Division, West’Virginia Pulp and Paper Company, 230 Park Avenue, New York 17, N. Y. (5) “Meadol” prepared from mixed hardwoods by Mead Corporation, Chillicothe, Ohio; and “Tomlinite” a soda lignin produced by Howard Smith Paper Company, Cornwall, Ontario. (6) R. F. Patterson, K. A. West, E. L. Lovell, W. I,. Hawkins and H. Hibbert, THIS JOURNAL, 63,2065 (1941). (7) C. P. Brewer, L. M. Cooke and H. Hibbert, ibid., 70, 57 (1948). ( 8 ) Experimental section of this paper. (9) F. E. Brauns, THrs JOURNAL, 61,2120 (1939). (10) M. A. Buchanan, F. E. Rrauus and R. L. Leaf, ibid., 71, 1297 (1949). ~~
~
the solubility parameter or &value with the square root of a substance’s cohesive energy density. It is defined by the equation 6 = (-
E/v’)’/Z
Where - E is the energy of vaporization to the gas at zero pressure and v1 is the molal volume of the liquid. Methods of calculation have been outlined and the values for many important solvents determined.3 Polymers in general exhibit a maximum solubility in solvents with &values closest to their own. I n Table I1 a series of solvents with little or no hydrogen-bonding capacity is listed in order of increasing solubility parameter. The ethanol spruce lignin fractions in the table were obtained by partial precipitation with petroleum ether from dioxane solution. (There seems to be adequate evidence that differences in precipitability and viscosity in lignin fractions primarily reflect differences in molecular weight as is the case with other polymers .6,1i) Clearly these lignin fractions are most soluble in solvents with &values around eleven, their solubilities decrease fairly regularly as the &value of the solvents decrease, and lignin fractions with increasing molecular weight are dissolved by solvents within a decreasing range of &values. Similar behavior has been observed with other polymers,3 for example, natural rubber, thiokol, polyvinyl chloride, polystyrene and polyisobutylene. For any considerable solubility of higher niolecular weight lignin fractions, it is necessary to use solvents with some hydrogen bonding capacity or basic strength, but by comparing solvents of similar basic strength, it is possible to observe the same dependence of solubility on solubility parameter with higher lignin fractions. For example the solubility of any of the listed lignins in dioxane, acetone, methyl ethyl ketone and ether decreases essentially with decrease in solubility parameter of the solvent. Gordy2 has shown that when hydrogen-bonding solvents are mixed with heavy methanol (CHsOD), a shift in wave length is observed in the oxygendeuterium band of the infrared region of the (11) C Schuerch, rbrd , 73, 3838 (1950) (12) G. Gee in “Advances in Colloid Science,” Vol 11, Iiiterscieucc Publishers, Inc., New Yorl,N. Y., 1946, p 186 B
VOl. 74
CONRAD SCHUERCH
5062
TABLE I THESOLUBILITY OF ISOLATED LIGNINSIN SINGLESOLVENTS PSolveut
Value, (cal./cc.) 1,’t
AP,O
microns
“Indulin” krsft C pine
“Meadol“ sodae hardwood
Hydro1 maple‘
Ethanol maplee
Native sprucec
Native aspenC
Ins Hexane 7.4 L Ins Ins Ins 8 9 Ins Ether Ins 7.5 Ins In5 6 6 0.19 Carbon tetrachloride 8.6 L Ins Ins Ins Ins Ins Xylene 8.78 L Ins Butyl Cellosolve 8.9 I1 Par Par Diethylene glycol 9.1 H Sol Sol Sli Ethyl acetate 0.12 Sli9.1 c Benzene 9.15 L Ins 7 I Ins Methyl ethyl ketone Par9.3 ParO.llh Chloroform 9.3 4 4 Sli Ins Chlorobenzene 9.5 0.02 Ins Ins Ins Ins Ins Ins Carbon disulfide 111. 0 L Ins Ins Dioxane 10.0 Sol Sol 0.14 Sol 3 3 Sol Sitrobenzene Sli 10.rr .04 Ins 0 Acetone 2 Sol Sol 10.0 .14 Par Par Alethy1 formate 10. 16 Parca. 0 12 ParAcrylonitrile 10.3 Sli CIL. 0 98 Sli I-Sitropropane Ins r, Ins 10.7 Pyridine 0 27 1 1 Sol Sol Sol Sol 10.7 Methyl cellosolve H Sol Sol Sol Sol Sol 10.8 Sol Sitroethane L Ins Ins 11.1 H Butanol-1 Ins Ins 11.4 H Ins Propanol-2 11.5 Sli Acetonitrile Sli 11.89 0.09 SIi Xtromethane 12.6 L Sli Sli 12.7 H SI i 5 5 Ethanol Sli Ethylene glycol 11.2 0.31 Sol Sol Sol Sol Sol SO1 Methanol 14.3 ParPar0.28 I-T In5 16.5 Ins Glycerol Water ir Ins Ins Ins 23.4 Ins 9 8 a Values from ref. 3 except for glycol derivatives and glycerine, the calculations for which are in the Experimental section of this paper. * Numeral values from ref. 2 except those labeled “cu.” which are estimates from Gordy’s data for analogous compounds. Where close analogies were not possible, H for high and L for low is used. The value for methyl ethyl ketone should perhaps be the same as that for acetone, cf. Gordy’s results with D20 in place of CHsOD. Ins = insoluble,PSli = slightly soluble, Par = partially soluble, Sol = soluble, numerals are in order of decreasing solubility for a given lignin. +
d
+
+
OF THE RELATION
Solvent
SOI.VEXI‘ &VALUE .4ND
&Value (cai./cc.!’/*
TABLE I1 LIGXINI I O L E C C L A R I 8
17
b7131CHT TO
1 6 s
ETHANOL SPRUCE LIGNINSOLUBILITY
Sample No.a 16A
I :3N
I 3A
I 1A
Ins Ins 111s Ins Ins Ins 611. 7 . 3 Ins Ligroin Ills Ins Ins Ills Ins Ins 8.6 Ins? Carbon tetrachloride Cl.15 Sli Ills Ins It15 Ills Ins Ins Benzene SliIns ItlS Ills Ins Itlh Ins Trichloroethylene 9 ,x I115 Ins Ins I115 Ins Ins 9.5 Sli Chlorobenzeiie Carbon disulfide 10.0 Iris Ins Ins Ins Ins 13s I115 Nitrobenzene 10.0 Sol Sol Sol Ins Ins Ins 111s ilcrylonit rile 10 . c-l Sol Sol Sol Sol Sol Sli Ins 1-Nitropropalie 10.7 Sol Sol Sli Ins Ins Ins Ins Xtroethane 11.1 Sol Sol Sol Par Sli Ills Ins Acetonitrile 11.9 Sol Sol Sol Sol Sol Par Ins Xitromethane 12.13 Sol Sol Sol Par SliIns Ins Roman numeral I represents the first ethanolysis of spruce woodineal reported in Experimental. Arabic numerals 1, 3 , 6, represent the three main fractions obtained by fractional precipitation of lignin I from dioxane by petroleum ether. Fraction I 8 was recovered from chloroform-ligroin solution after the original precipitation of ligroin-insoluble lignin. Fraction I 7 remained in solution after the precipitation of I 1 to I 6. The samples are therefore arranged above in order of increasing molecular weight unless previous work (reference 6 and 11)is completely misleading. The letters A and N represent alkalisoluble and “neutral” fractions from the partitioning of fractions I 1, I 3 and I 6. Some difference in solubility is apparent between the subfractions I 3A and I 3 N and between I 6A and I 6N. +
spectrum. The shift in wave length (Ah) is proportional to the hydrogen-bonding capacity or basic strength of the solvent. Of the solventsa tested by Gordy, those having Ah values of less than 0.14
micron are poor to mediocre lignin solvents, Le., nitro compounds, nitriles and esters. Those having Ah values of 0.14 or higher and solubility parameters around eleven are good lignin solvents.
Oct. 20, 1952
SOLVENT PROPERTIES OF LIQUIDS:RELATION TO FRACTIONATION OF LIGNIN
Table I11 lists seven solvents whose solubility parameters are between ten and eleven or close t o TABLE 111 THEEFFECT OF HYDROGEN BONDING CAPACITY ON LIGNIN SOLUBILITY Solvents listed in order of increasing solvent power for lignins &Value (cal./cc.)'/¶
1 Carbon disulfide 2 Nitrobenzene 3 Acrylonitrile 4 Methyl formate 5 Acetone 6 Dioxane 7 Pyridine
10.0 10.0 10.5 10.16 10.0 10.0 10.7
4,
micron
Low 0.04 ca. 0.08 ca. 0.12 0.14
0.14 0.27
the optimum value for lignin solubility. Their ability to dissolve lignin increases regularly from carbon disulfide, a poor lignin solvent, t o pyridine, a powerful solvent, or in the order of increasing basic strength. The same effect is noticeable a t other values of solubility parameter. Ether is, for example, a more powerful lignin solvent than carbon tetrachloride, xylene or benzene. The glycol ethers are excellent lignin solvents because of their hydrogen-bonding capacity although they vary rather widely in solubility parameter; other examples could be cited. Chloroform, the hydrogen of which has acidic properties, is a better solvent than chlorobenzene, a fact which suggests that lignin can act as a proton acceptor as well as donor. If a plot of solvent &value vs. solvent hydrogenbonding capacity is made, good lignin solvents will be found in a roughly parabolic area, the apex of which will be a t a solubility parameter of around eleven and a Ap value dependent on the molecular weight of the lignin sample.lZa I n a homologous aliphatic series such as the alcohols (and perhaps the nitro compounds) the solubility of lignin decreases abruptly with increasing hydrocarbon chain length, even when the solubility parameter is closer to a value of eleven in the higher members. The effect appears similar to the diluting of a good solvent with an aliphatic hydrocarbon, and the higher alcohols are poorer solvents than would perhaps be expected from their solubility parameter and the presence of a hydroxyl group in the molecule. The high association of pure alcohols frequently results in a lower solvent power than expected2l3; and the assumption that the molecules have a uniform force field around them, which is implied in the use of the &value, is clearly not true for molecules having non-polar and highly polar portions.12 Maple ethanol6 and hydrol" lignins have been separated into fractions of differing viscosity and molecular weight by pouring solutions of the polymers into solvents of varying 8-value and basic strength. As would be expected from the foregoing discussion, water and petroleum ether have been found to dissolve primarily monomers; ether, benzene, ethanol and chloroform separately (12a) NOTEADDRD IN PROOF.-A similar treatment of polymer solubility by H. Spurlin has just come to our attention in J. Polymer Sci., 3, 714 (1948).
5063
dissolve intermediate molecular weight fractions as well, while dioxane, acetone and (especially) pyridine are the best individual solvents for all molecular sizes. These separations again reflect the fact that low molecular weight lignin fractions dissolve in solvents with a *wider range of solubility parameters, and a weaker hydrogen-bonding capacity than do the high molecular weight fractions. Table IV lists the solubility of a soda lignin in mixed solvents. Whenever a hydroxylated solvent, such as water or the lower alcohols, is mixed with a solvent of lower hydrogen-bonding capacity and a &value of nine or greater, the solubility of lignin is greater in the mixture than in either solvent alone. I n all cases the hydrogen-bonding capacity of the mixture is greater than that of one individual solvent and in most (though not all) cases the solubility parameter of the mixture is presumably closer to the optimum than with either solvent. The effect is noticeable even on the addition of water or alcohols to dioxane or acetone but is striking with a mixture of two poor solvents like nitroethane and butanol, or benzene and ethanol. The cellosolves, however, are such effective solvents that no qualitative improvement was noted on mixing them with other liquids. Solvents with &values less than about nine seem t o act as simple precipitants and the solubility of lignin in mixtures which include them always appears to be less than in their absence. Apparently the most extensive data on lignin solubility are contained in a list of about 170 liquids and 60 binary mixtures that have been tested as solvents for commercial pine kraft lignins4 These data show the same trend of dependence on 6value and hydrogen bonding-capacity that has been outlined above and also show that the bulk of kraft lignin has a solubility comparable t o the highest molecular weight fractions of ethanol and hydrol lignin. We have found no evidence that any other properties of the solvents are of comparable importance in determining lignin solubility. Swelling experiments on insoluble K l a ~ o n , l ~ ~ W i l l ~ t a t t e r , 'periodate,I3c ~~ and wood hydrolysis14 lignins or-more pertinently-lignin in wood have in the present research not been carried out, but the swelling of cuproxam lignin increases in about the same order as the solubility of other lignins, Le., water < benzene < methanol < ether < acetone < pyridine13d; and moist phenol and formamide, both excellent solvents for other isolated lignins, are good swelling agents a t least for periodate spruce lignin.15 Similarly the position of maximum swelling of rubber when crosslinked b y different methods and to different extents is a t the same value of solubility parameter as is the maximum solubility of the native polymer.12 The insolubility of lignin as it exists in wood has received a t one time or another most of the ex(13) F. E. Brauns, "The Chemistry of Lignin." Academic Press, Inc., New York, N. Y.,1952; (a) p. 55; (b) p. 57; ( c ) p. 63; (d) p. 206; (e) p. 211; (f) p. 646. (14) "The Madison Wood-Sugar Process," United States Department of Agriculture, Forest Service Report No. R 1617. Forest Products Laboratory, Madison 6. Wisconsin. (15) Pulp and Paper Research Institute of Canada, Montreal, Canada, Fall Quarterly Report (1947).
Vol. 74
5064 T.4BLE I \ THESOLUBILITY or A SODA LIGSIS “TOMLISITE” MIXEDSOLVEXI 5‘‘
Benzene Chloroform Chlorobenzene Dioxane Nitrobenzene Acetone Methyl formate Acrylonitrile Methyl Cellosolve Nitroethane Butanol Nitromethane Ethanol Wateru
9.15 0.3
L
9.5 10.0 10.0 10.0 10.16 10.5 10.8 11.1 11.4
0.02 H 0.04 0.14 c n . 0.12 cli. 0 . OS II I, Ii 1.
12.6 12.7 23 .I
..
FI FI
Ins Ins Ini Par Ins Par -b SI1 SI1 Par
sii Par s11 Par Ins
+
Ins 76 Ins Sol Ins Par Sli Sli Sol Sli Sol Sli Sol Ins’
Ins Par Ins Sli Sli Sli Sol Sli ParT Sli Sol Ins
Sol Sol Sol Sol Par Sol Sol Par Sol
+
Sol S1in
Sli Sol Sli Sli Sol Sli Sol Sli Sol Sli
solSlit Sli+ Sol Sli Sol Sli’ Sol Sol-
Sll’ Sli Sol SliPar Sli Par
Par
Sli so1
SliSol SliSol Sli
Sol Sol SolSol Sol Sol
Sli-
Sol Sli
Sol Ins
InsSol Ins Ins
Sli Sol Ins
Sli In,
Ins
Equal volumes of the two solvents used. The solubility of lignin in dioxane is, however, enhanced by small amounts of water. The solubility of lignin in water-immiscible solvents is little affected by the presence of water. Plus and minus signs indicate solubilities somewhat greater or less than other cases described by the same word ;.e., insoluble, slight, partial soluble. a
planations applicable to an organophilic polymer system interpenetrated with carbohydrate impurities. The correct interpretation of its insolubility is of importance in the choice of method of structure investigation. I t appears pertinent, therefore, to review briefly the more frequent speculations in this field. A highly Crystalline structure is ruled out for lignin by X-ray diffraction studies.13e A chemical linkage to cellulose appears improbable since electron micrographs show lignin to have an amorphous granular structure that exhibits no intimate connection with cellulose,16and since the crystalline character of cellulose would seem to limit such a union to faults or fissures or on surfaces.” Such a superficial linkage, if it were to exist, is not sufficient reason by itself for lignin insolubility since the removal of carbohydrates by periodate oxidation and neutral hydrolysis leaves behind an insoluble lignin. l8 A linkage to the hemicelluloses also seems inadequate for these too are removed by periodate oxidation; most of the hemicelluloses are also removed by the peptizing action of dilute alkali in the cold while most of the lignin remains insoluble. I t is improbable that lignin is inaccessible to swelling solvents since fine subdivision does not affect the yield of ethanollg or native20 lignin, and wood density has little effect on the rate of alkaline pulping.21 Lignin is apparently also accessible to sulfite liquor.22 Excessive entangling with the carbohydrate fraction does not explain lignin insolubility since lignin shows no evidence of chain structure, and the known interpenetration of hemi( 1 6 ) K. Muhlethaler, Bioclriiii. el Biophrs. Acta, 8, 15 (1949).
(17) C B. Purves i n I;. Ott’s, “Cellulose and Cellulose Derivatives.” 1943, p. 101. Interscience Publishers, I n c . , N e w York, N. Y., 118) W. J . m’dld, P. 1;. Ritchie a n d c H. Purve5, THISJ O U R N A L , 69, 1371 (1947). (19) W. B. Hewson. .I I,. McCarthy and 13. Rihhert, ibid., 63, 3041 (1941). (20) F. E. Brauns and H. Seilen, T a p p i , 85, 67 (1932). The authors list t h i s fact and others as evidence of a lignin-carbohydrate linkage or a condensed lignin structure. (21) G. r,. Larocque arid 0. Maass, Cali. .T Research, B16, 89 (1937). ( 2 2 ) J hf Ctilhriiin l? 11. Torston iiiiil 0. S l a n i s , i h i d . , B17, 11’1 (1939)
celluloses with lignin16 does not prevent their separation. The preceding analysis of lignin solubility suggests that there are no highly specific solvent requirements for lignin, but rather that its observed solubility behavior is that to be expected of an aromatic polymer with many hydroxyl groups. There remains then only the most obvious explanation of lignin insolubility : that lignin has a three dimensional structure. The solution of a network polymer requires a fragmentation by chemical reaction, as does lignin. Fragments obtained by random cleavage would be expected to be of various ~izes,6*~ relatively low molecular weight, 13e chemically heterogeneous, and in part non-linear.24 This is the case with all isolated lignins. In addition in one isolation method the known chemical changes occurring on the lignin can be written as a balanced equation of so1volysis.l While a lignin carbohydrate complex cannot be completely excluded, i t does not appear t o be a complete explanation for the insolubility of lignin and the independent evidence for cross linked structure(s) seems impressive. One attack on the lignin problem has been the search for a method of isolating the material chemically unchanged. If however lignin is three-dimensional, it is of equal importance to have available a method of degradation of proven and preferably uncomplicated mechanism. In this connection, a study of the acid-catalyzed solvolysis of lignin under conditions advantageous for its solution may have some merit : Only two reagents are required, inaking i t perhaps possible to interpret the reaction mechanism more readily. The hypothetical reaction previously mentioned can serve as a theoretical basis of investigation. There are probably no important methods of dissolving lignin which do not require the action of either hydrogen or hydroxyl ions. Cyclizationzs and intermolecular condensationsz6 11123
(23) H. Pauly, Bcr., 67, 1177 (1934). (24) H. Staudinger and E. Dreher, ibid., 69, 1729 (1936). ( 2 5 ) € I . Richtzenhain, ibid.. 83, 488 (1950). (28) E; A V’~es1.\T I,. l I a w ~ i ~ i s a 11. u ~ Il l i i ~ l ~ ~ r t J, O’ L~KrH~A I~, . s63, 3038 (1941).
Oct. 20, 1952
SOLVENT PROPERTIES OF
LIQVIDS:R E L A T I O N
have received considerable attention as side reactions occurring during alcoholysis and are important complications which require further assessment. If occurring to only a small degree they may not vitiate structural conclusions. I t is perhaps pertinent that eugenol has been found extremely resistant to acid-catalyzed condensation with aliphatic and aromatic aldehydes” and that monomers isolated from lignin resist resinification by ethanolysis when present in high dilution.26 A repolymerization reaction must be dependent on the square of the substrate concentration, while depolymerizations need not be. The lignin concentration in the following preparations was about 1% or less and repolymerization should therefore be minimized. For exploratory work the ethanolysis reaction was chosen and of the variety of solvent mixtures superior to pure ethanol, the combination of chloroform and ethanol was used. X series of standard ethanolyses was first carried out,6 with varying proportions of chloroform in the solvent mixture. Extent of delignification was measured by Klason lignin determinations. A typical delignification maximum was observed a t close to an azeotropic mixture of the two solvents (Table V). Under conditions which gave about 30% delignification with ethanol as solvent, chloroform-ethanol (80 :20) gave about 75% delignification.
TO
FRACTIONATION OF LIGNIN
5065
was as great as that of any other lignin preparation. Table VI summarizes the data obtained on the ligroin insoluble material obtained by this three-step ethanolysis. The following points seem of interest : The viscosities of the lignin solutions were low and the plot of the ratio of specific viscosity t o concentration (.VSp)/(c) against concentration was linear over the range studied from 1.5 to 4 g. per 100 ml. The intrinsic viscosity of the ligroininsoluble products increased from the first to the second to the third ethanolysis. The first ethanolysis also gave rise to the largest quantity of ligroin-soluble material. An analogous result has been observed with sulfite pulping, another acidcatalyzed lignin depolymerization. Cooks which remove only small amounts of lignin produce soluble lignosulfonic acids of low molecular weight, while waste liquors from cooks removing large proportions of the lignin contain largely sulfonic acids of high molecular weight. The values of the constant k’ calculated from the equation C
+ k’[n]ZC
= [n]
were not the same for the three products as would be expected in a homologous seriesz9 but were nearly the same for the first and third ethanolysis, markedly greater for the second. The proportion of and alkali-soluble TABLE ir material was determined in each fraction by a VARIATIONIN YIELD OF ETHANOL LIGNININ DIFFERENT three stage (four separatory funnel) countercurrent CHLOROFORM-ETHANOL SYSTEMS~ extraction between aqueous ethanolic alkali and Original Klason lignin lignin ethanolic chloroform. The fraction subjected to dissolved, Ethanol, Pulp, content of ethanolysis for the longest period of time and No. % yield. % Pulp, % 70 having the highest viscosity also had the highest 21.2 41 10 77.2 I proportion of alkali-soluble material. This result 57.3 16.1 66 I1 15 is not consistent with a single structure for lignin 51.4 12.0 77 20 I11 in which the phenolic oxygens form a portion of a 40 59.2 18.3 60 IV linear unbranched chain free of heterocyclic 62.7 21.0 52 50 V rings 13f but is consistent with almost any more 69.6 24.5 38 vI 80 complicated system. 100 73.3 25.8 32 VI1 The three main products were fractionated a Norway spruce wood meal heated 49 hours at 64” with according to molecular weight by partial precipi1.76% hydrogen chloride. tation, similar fractions were recombined to give It also seemed of interest to determine whether larger quantities and the different molecular weight lower concentrations of mineral acid were feasible bands were distributed between alkali and chlorofor etfianolysis. I n a preparative experiment, form. Even when comparing fractions subjected therefore, the concentration of hydrogen chloride to ethanolysis for the same length of time, the was decreased from 1.75% used in the previous lower molecular weight fractions had higher proseries t o 0.7% (0.2 N). Following reaction, the portions of “neutral” material than the high moleclignin was isolated by filtering off and washing the ular weight fractions (Table VI). This result was wood pulp, washing the combined chloroform obtained with samples I and I1 but could not be liquors with water, drying, concentrating in vacuo confirmed with sample 111,for the highest molecular and precipitating into petroleum ether. Three weight fraction precipitated from chloroformconsecutive ethanolyses of 40-hours duration were ethanol on the addition of the aqueous ethanolic required to give a total yield of 73y0 delignifica- alkali and set t o an insoluble product. Apparently tion (Table VI). The rate of delignification was higher dilution and a better aqueous solvent system obviously substantially lowered by a decrease in is required for the partitioning of higher molecular weight fractions in order to avoid insolubilization. hydrogen ion concentration. Although these products were obtained by re- The observed concentration of the so-called “neuaction a t a temperature about 15 degrees lower than tral” fraction in the lower molecular weight por(28) H. Erdtman, Svcnsk Pagpeuslidn, 46, no. 16, 315 (1942); usual, and a t about one-third the concentration I m l . Paper Chcm., 13, 79 (1942-1943). of mineral acid, the heterogeneity of the products Bull. (29) C. E. H. Bawn, “The Chemistry of High Polymers,” Inter(27) A. Michael a n d J. P. Ryder, Am. Chcm. J., 9, 130 (1887).
science Publishers, Inc.. New York, N. Y., 1948,p . 166.
I‘d.i-l
CONRAD SCHUERCH
5066
TABLE VI SPRUCE ETHAXOL LIGNINPREPARATIO~
200 g. Sorway spruce wood irieal with 27.67, or 55.2 g. Klasoii lignin Y
EtOH-HCl in CHCL
-+
\Toot1 residue
--+
EtOH-HC1 in CHClt
--+
\\.ood residue
,+
-1 ,+
Ligroiii,’ soluble
i Fractional I
Fractional
precipitation
J.
+
105 g. n.ood residuc with 14.7c, or 13 4 g . Klason lignin
Ligroin insol. Ethanol lignin I11 7.1 g., 84% alkali solubleb; [ n ]= 0.0626
Ligroin insol. Ethanol lignin I 11.1 g., 65% alkali solubleb; In1 = 0.0477
~
EtOH-HCl in CHClt
1 Soln. -1 Ligroin“ $. soluble
, Solution, pptd
into ligroin Ligroin” ‘y soluble
--+
1
+
I precipitation
+
Fractional precipitation
m
j. Y Low mol. wt. Med. mol. wt. High mol. wt. Low mol. wt. High mol. wt. Low mol. wt. High mol. wt. 85% alkali79% alkalipartitioning unsuccessful si“.’, alkali53% alkali51% alkali70% alkalisol. sol.( 501. sol. so1.c sol.“ The “ligroin soluble” fractions were combined and weighed 11.0 g. Total yield of all products was 56.3 g. of lignin from 55.2 g. originally present in the wood. a Per cent. alkali soluble was calculated as 70recovered in separatory funnels A and B divided by total recovery, and the quotient multiplied by 100 (see Experimental). Weighting these results by quantity of fraction recovered gave a total yoalkali solubles equal to 57% or slightly lower than that found on the unfractionated material (65$Z0).
tions of spruce ethanol lignin is the reverse of the distribution obtained with maple hydro1 lignin. Whether this difference is due to extent or method of degradation, difference in wood source or some other reason must await further investigation. The fact that previously maple ethanol lignin has been obtained in higher yield than spruce ethanol lignin3* is apparently due to the greater solubility of the former in ethanol. The greater solubility is to be expected of a more heterogeneous polymer even if of the same molecular weight and need not a t present be ascribed to a greater tendency of spruce ethanol lignin to repol?-merize.
Experimental Solubility Tests.-Small samples (0.1 g.) of lignin were shaken a t room temperature with 10 ml. of individual or mixed solvents (1 : 1 by volume) and observed for depth of color of solution and for the presence of undissolved material. The results in some cases indicate less solubility than recorded in reference 4 and in general minimize the solubility of the lignin samples in order t o emphasize differences betneen solvents. Ethanolysis with Different Solvent Proportions .-Fivegram samples of air-dried Norway spruce meal (40-80 mesh, ethanol-benzene extracted) were sealed in tubes with 40 ml. of chloroform-ethanol in various proportions containing 1.76% hydrogen chloride. The tubes were rotated in a 64’ oil-bath for 49 hours. The wood residues were filtered off, washed four times with about 50 ml. of their own solvent mixtures, and the liquors and washes made up to 100 ml. The liquors from tubes 1t o 3 (with high chloroform content) were clearly the darkest. The wood residues varied in color from almost black (No. I ) to light gray (No. 111) to almost unchanged (No. VII). The Klason lignin contents and D U ~ Dvields were at a minimum with No. I11 and are listed i n y a b l e V. Preparation of Ethanol Spruce Lignin in a Mixed Solvent. --‘Two hundred grams of air-dried Norway spruce meal (30) L. Brickman, J. J. Pylt, J. L. AZcCarthy and H. Hibbert, Tx18 61, 868 (1939)
JOURNAL
(40-80 mesh, ethanol-benzene extracted, 27.6% Klason lignin) was refluxed with 0.2 iV hydrogen chloride in 1,930 ml. of chloroform absolute ethanol (8:2 by volume) under carbon dioxide. After 43 hours, the wood residue was separated, washed with chloroform-ethanol and treated with fresh reaction solution for two more periods of about 43 hours. The reflux temperature was about 60”. The solutions and washings from each ethanolysis were washed with 5% bicarbonate and with water and were dried over magnesium sulfate. The three liquors were individually concentrated to about 100 ml. under reduced pressure (20-25°) and precipitated into ligroin. The ligroin-soluble oils (largely from the first ethanolysis) were combined and weighed 11.0 g. The ligroin-insoluble ethanol lignins, I, I1 and 111, were aai4hed with ligroin and isolated separately as light colored powders. A lignin balance is given in Table VI. A sample of these lignin fractions was combined and distilled with 12% hydrochloric acid after the “Tappi” standard pentosan determination. The distillate from one gram of lignin absorbed 0.2 milliequivalent of bromine, the equivalent of 1.37, pentosan, without correction. The ethanol lignin samples were analyzed for chlorine and contained less than O.;j%. Lignin Partitioning.-Thirty grams of sodium hydroxide was dissolved in 1000 nil, of 48% ethanol. This solution on equilibration with 430 ml. of chloroform gave 750 ml. of a chloroform layer arid about 670 ml. of an aqueous phase. Approximately 1-g. lignin samples were partitioned between 25 ml. of each of these two phases and the phases transferred and equilibrated in a countercurrent fashion as previously23 according to Craig’s technique. Four separatory funnels were used and in each case the bulk of the lignin appeared in the first “A” (alkali-soluble) and in the fourth “N” (chloroform-soluble) fractions. After partitioning, the liquid layers were acidified with sulfuric acid, the layers separated and the water extracted with 1 ml. of chloroform. The chloroform layers were dried, concentrated in vacuo to less than 5 ml. and precipitated into a small centrifuge cup filled with petroleum ether. The precipitates after washing were recovered in the percentages listed in Table VII. !31) L. E. Wise, “Wood Chemistry.” Reinhold Publishing Corp., New York, N. Y., 1944,p. 622. ( 3 2 ) L. C. Craig, C. Golumbic, H. hfighton a n d E. Titus, J , H i d Chcm., 161, 321 (1945).
Oct. 20, 1952
SOLVENT PROPERTIES OF LIQUIDS:RELATION TO FRACTIONATION OF LIGNIN
5067
transferred to a new centrifuge cup. The lignin fraction was redissolved in a minimum amount of acetone-ethanol SEPARATION OF ALKALI-SOLUBLE AND NEUTRALLIGNIN and precipitated into a small centrifuge cup of ligroin, centrifuged, washed with fresh ligroin and air-dried. Six fracFRACTIONS tions were obtained and recombined to give two or three Recovered in funnel, yo Total main fractions, the first precipitating in each case being Wt. in recovLignin sample g. A B M N ered called “high mol. wt.” in Table VII. Only in the case of lignin I are the three fractions essentially the equivalent of Lignin I unfract. 1.0 49 8 . . 31 88 the unfractionated material. In the case of lignin I1 the Lignin I1 unfract. 1.0 5 5 20 86 56 high molecular weight fractions were not all recombined and Lignin I11 unfract. 1.0 6 4 11 94 73 the result of partitioning the fractionated material is not Lignin I1 high mol. wt. 0.9 51 6 5 19 81 completely comparable with the unfractionated material. Calculation of Solubility Parameter.-The solubility Lignin 1 3 med. mol. wt. 1.27 36 4 6 32 78 parameters of several glycol derivatives were calculated Lignin ISlow mol. wt. 28 3 5 22 1.2 58 from data obtained from the Carbide and Carbon Chemicals Lignin I1 high mol. wt. 0.85 81 2 4 10 97 Company and listed in Table IX. The following formula Lignin I1 low mol. wt. 1.05 38 8 81 6 29 was used Lignin I11 low mol. wt. 1.0 61 4 5 12 82
TABLE VI1
Viscosity Determinations.-One-gram samples of ethanol lignins I , I1 and I11 (Table VI) were dissolved in 25 ml. of dioxane, filtered through glass and their viscosities determined at 20” in an Ostwald viscosimeter. Five duplicate readings were obtained and 20 ml. of the solutions diluted again to 25 ml. In this fashion readings a t five dilutions were obtained. The time of efflux for pure dioxane was 129.3 seconds. The values of N,,/Care listed in Table VIII.
+ 0.002146C; lignin 11, Xnp/c = 0.0502 + 0.004649C; lignin 111, iV.,/c = 0.06259 + 0.003815C;
correlation coefficient = 0.949 correlation coefficient = 0.958 correlation coefficient = 0.954
Solving for k‘ in the equation
lig I, k’ = 0.943 lignin 11, k’ = 1.845 lignin 111, K’ = 0.974
Sample
FOR 4%
= heat of vaporization in cal./mole = b.p., OK. s.g. = specific gravity M.W. = molecular weight
T
TABLE IX
The lines of best fit are lignin I, N,,/C = 0.0477
VALUESOF N.,/C
where AH
TABLE VI11 UNFRACTIONATED ETHANOL LIGNINS’ 3.270
2.66%
2.05%
1.64%
Lignin1 0.0548 0.0516 0.0518 0.0498 0.0495 ,0692 Lignin I1 ,0642 .0616 .0618 .0567 ,0783 Lignin I11 ,0740 .0718 .0722 .OW0 Concentration expressed in g./lOO ml. solution. See reference 29 for details. Fractional Precipitation of Lignin.-Three- or four-gram samples of lignin I , I1 and I11 were separately dissolved in acetone-ethanol (9: 1) (or less advantageously in dioxane) to make 10% solutions. Each solution was stirred in a 250ml. centrifuge cup and ligroin was added dropwise. On precipitation of a suitable fraction the mixture was stirred for about ten minutes, centrifuged, and the supernatant
DATAFOR
THE
CALCULATION OF SOLUBILITY PARAMETER 6
Compound
Ethylene chlorohydrin Ethylene glycol Diethylene glycol Cellosolve Methyl Cellosolve Butyl Cellosolve
Heat of evaporation ~ p . g . B.t.u./lb. 2Oo/2O0
221 344
1.2045 1.1155
150 234
1.1184 0.9311
239 171
.9663 .go19
M~I. wt.
80.5 62
Value in ~ . p . (Gal./ OC.’ cc.)lly
128.8 11.6 197.2 14.2
106 244.5 9.1 9 0 . 1 135.1 10.6 76.1 124.3 118.2 170.6
10.8 8.9
Cal./mole a t 55O 5O0/4O
Glycerol 21,060” 1.242 92 . . . 16.5 ‘From D. F. Stedman, Trans. Faraday SOL., 24, 296 (1928).
Acknowledgment.-The author is greatly indebted to Dr. R. S. Mesrobian of Brooklyn Polytechnic Institute for introducing him to the use of the solubility parameter and for criticism of this manuscript and to Dr. 0. L. I. Brown of Syracuse University for helpful discussions on the same subject. SYRACUSE 10, NEW YORK
